Added equations to the docstring of seis.wavefd2d
Added equations to the docstring of seis.wavefd2d

santis19/fatiando,eusoubrasileiro/fatiando,eusoubrasileiro/fatiando,eusoubrasileiro/fatiando_seismic,mtb-za/fatiando,eusoubrasileiro/fatiando_seismic,victortxa/fatiando,santis19/fatiando,cmeessen/fatiando,rafaelmds/fatiando,drandykass/fatiando,fatiando/fatiando,fatiando/fatiando,victortxa/fatiando,eusoubrasileiro/fatiando,cmeessen/fatiando,mtb-za/fatiando,rafaelmds/fatiando,drandykass/fatiando